structural frame. The 3D modeler has no clue about (and probably doesn't
care to know about) where the CG is - and that's fine. The FDM and the 3D model, though, *do* need to agree on a common MRP (Model Reference Point) that the FDM can supply to the FlightGear scene code for proper placement of an aircraft. Once that point is agreed upon, it's not a big problem at all for the FDM to send to FlightGear the exact location in world space of the MRP. No matter what the orientation of the aircraft, the 3D model will always have its CG in the correct place if the FDM properly reports to FlightGear the real-world location of the MRP. We (FDM) can do this because, as I've said before, we know where the CG is, and we know where the MRP is in reference to the CG.
I see what you're doing now. You are letting them just use the nose, and then shifting the FDM nose point until the FDM center is near the visual center.
The nose reference point in the visual model isn't referenced and matched to the FDM. It is simply a given point, and then the FDM can be adjusted to it until it's COL matches the visual's approximate COL. That works easily but is different than the model's nose point being matched to the FDM. The 'Model Reference Point' terminology is what is actually ambiguous, it is really just a visual reference point that the FDM is adjusted for after the fact.
Handy dandy way to match the visual to the FDM really, but I would have shied away from calling it 'Model Reference Point' without it being the best inertial reference point of the model. It can be considered the reference point of the visual model frame, but since it's going to be translated through by the FDM the actual visual point of motion center will be the same as in the FDM. It's an unusual naming for just the point of visual adjustment.
Alan
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